What is most interesting about a slip of professionalism like this is what they reveal about the teacher. We all had teachers who lost it with a class and said something stupid. Maybe they swore, maybe they made a Freudian slip, maybe they were violent. What Sonsby is reported to have said was: “I am so bloody tired of you Muslims ruining lessons.”

I think the “you” is the most worrying word in the whole sentence. If she had said “tired of Muslims”, this would have been unprofessional, but “you Muslims” seems very much intended as an insult. Her school has a large percentage of Muslim students and only a handful of them cause trouble. This slip of the mask gives the impression she thinks of this background as a negative, even when the child is well behaved and respectful. With that sort of mindset, is it a surprise that her school has a discipline problem among the children from the very background she undervalues?

Children pick up on their teachers’ expectations. I remember one fascinating study in which some teachers were falsely told that their average ability students were gifted. Lo and behold, they then began to observe increases in the children’s abilities. People of all ages respond to ‘stereotype threat’. If one has absorbed a certain message – like “girls are bad at maths”, for example – they will respond by living up, or rather down, to that stereotype. It has been proven that girls do worse in maths tests after reminders that they are female (and do better when they are not reminded of this!).

If a teacher believes that ‘all Muslims’, ‘all children of alcoholics’, or ‘all handball players’ are a certain way, then they will inevitably find out that they are right. If society further exposes children to the view that certain children are worthless, criminal, intelligent or kind, it becomes a self-fulfilling prophesy.

Children are incredibly sensitive to the messages adults send them.

Danish popular culture often reinforces negative stereotypes about children and teens from certain backgrounds. This is normalised and presented as plain talking, rather than prejudice. I met a man who told me he would never send his kids to “one of those ghetto schools”. Then he said that this was not racism because the “they” he referred to are a social group.

Plenty of ‘ghetto’ schools demonstrate that with good teaching and high expectations, they can get the very best out of their students. In Fredericia, the school in our ‘ghetto’ has very good results, particularly in English. This is because the teaching team are fabulous and dedicated. They show respect to their students, and the students respond in kind. Research from the Achievement Gap Initiative at Harvard University shows that better teaching and leadership are what raises standards, not blaming students or their upbringing.

Many armchair school improvement experts suggest that ‘spreading them out’ is the answer. This suggestion speaks only to their lack of higher cognitive ability. An easy quick fix is tempting for its simplicity, but unintended consequences are inevitable. Treat a group of people like animals, or tell them they cannot choose a school for their child because of their social group, and the harm is long-lasting. Research from other countries, not least the US, bears this out.

Schools with challenging students can be the best schools in their area. The way to achieve this is far from simple. It includes strong management, high expectations, teacher accountability and excellent teaching. There are many schools that are exemplary despite what the community thinks of them because of their student make-up.

Sonsby may be a good teacher. She might be a good school leader – she won an award for successfully merging two schools into one after all. But without full and critical reflection on her leadership – particularly on her prejudices regarding her students and the potential harm these ideas cause – she will never have a school that doesn’t have a hardcore group of lesson ruiners.